| Summary: | Trabajo presentado en el III Simposio Internacional en Ciencias del Mar (ISMS12), celebrado en Cádiz del 24 al 26 de enero de 2012. The distribution and composition of the phytoplankton assemblages, around the South Shetland Islands (SSI, Antarctica), was analysed during January 2010 in relation with the physicochemical variables and the maximum photosynthetic efficiency (Fv/Fm) measured with a Fast repetition Rate fluorometer. Nanophytoplankton (2 - 20 μm) was the most abundant phytoplankton fraction with significant differences among waters from the Drake Passage, the SSI shelf, the Bransfield Strait, and the Antarctic Sound. A latitudinal pattern in Fv/Fm along a transect that crossed the SSI occurred, ranging from 0.17 to 0.53, indicating that cells were in good physiological conditions in the area surrounding the SSI and the Bransfield Strait but start to get stressed at the stations located further north where the smaller phytoplankton group (‘Nano small’, 2.8 μm mean ESD) was detected. A deeper examination carried out according to the sampling time and the vertical depth profiles detected surface photoinhibition processes and regional trends. At surface significant differences were found among stations sampled at different day time, irradiance and silicate concentrations. The most abundant phytoplankton group in surface waters was that of small diatoms (‘Nano medium’, 5.2 μm mean ESD), probably due to a better photoadaptation against the rest of the groups. At the Depth of the Fluorescence Maximun (DFM), phytoplankton showed homogeneous values of Fv/Fm close to the maximum, regardless time of the day and group. In conclusion, the influence of the water masses together with irradiance and the position in the north-south gradient of limiting nutrients were driving the physiological state of the different phytoplankton groups around the SSI. Peer Reviewed
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